The heart includes a number of normal pathways that are responsible for the propagation of electrical signals from the upper to lower chambers necessary for performing normal systole and diastole function. The presence of an arrhythmogenic site or accessory pathway can bypass or short circuit the normal pathway, potentially resulting in very rapid heart contractions, referred to here as tachycardias.
A variety of approaches, including drugs, implantable pacemakers/defibrillators, surgery, and catheter ablation have been proposed to treat tachycardias. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying causes. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem, usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. It is important for a physician to accurately steer the catheter to the exact site for ablation. Once at the site, it is important for a physician to control the emission of energy to ablate the tissue within the heart.
Of particular interest to the present invention are radiofrequency (RF) ablation protocols that have been proven to be highly effective in tachycardia treatment while exposing a patient to minimal side effects and risks. Radiofrequency catheter ablation is generally performed after conducting an initial mapping study where the locations of the arrhythmogenic site and/or accessory pathway are determined. After a mapping study, an ablation catheter is usually introduced to the target heart chamber and is manipulated so that the ablations tip electrode lies exactly at the target tissue site. Radiofrequency energy or other suitable energy, such as ultrasonic energy, laser energy, or microwave energy, is then applied through the tip electrode to the cardiac tissue in order to ablate the tissue of the arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signal patterns responsible for the tachycardia may be eliminated. However, in the case of atrial fibrillation (AFib) or atrial flutter (AFlu), multiple arrhythmogenic sites and/or multiple accessory pathways exist. The conventional catheter with a single short ablation electrode can not effectively cure the symptoms.
Atrial fibrillation is believed to be the result of the simultaneous occurrence of multiple wavelets of functional re-entry of electrical impulses within the atria, resulting in a condition in which the transmission of electrical activity becomes so disorganized that the atria contracts irregularly. Once considered a benign disorder, AFib now is widely recognized as the cause of significant morbidity and mortality. The most dangerous outcome from AFib is thromboembolism and stroke risk, the latter due to the chaotic contractions of the atria causing blood to pool. This in turn can lead to clot formation and the potential for an embolic stroke. According to data from the American Heart Association, about 75,000 strokes per year are AFib-related.
The tip section of a conventional electrophysiology catheter that is deflectable usually contains one large rigid electrode about 4 to 8 mm in length for ablation purposes. Because of the rigidity of the large electrode, the length is severely restricted. Sometimes, a plurality of long electrodes is used in creating a contiguous, non-continuous, linear lesion. In some clinical trials, the gap between two lesions is so large that it is not even considered as "contiguous".
Recently, a catheter system having a coil-type electrode has been used clinically. Though a coil-type electrode can be very long as a result of its flexibility properties, the resulting lesion from such a catheter is at best contiguous. The outermost ridge of each coil pass of the coil-type electrode contacts the tissue, wherein the distance between the ridges may only create a contiguous linear lesion. Furthermore, the overall outer surface of the catheter having a coil-type electrode is wavy or bumpy. Blood clotting has been observed at a low-flow spot of the coils, such as the hind side of the coils with respect to the bloodflow direction.
While a radiofrequency electrophysiology ablation procedure using an existing catheter has had promising results, the tip section of a known catheter usually has a rigid fixed-length electrode when contacting the tissue for ablation purposes. Therefore there is a need for an improved catheter and methods for making a linear and larger lesion in the cardiac tissue employing a flexible "long" electrode during ablation procedures.